[time-nuts] ADEV vs MDEV

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Feb 6 17:32:55 EST 2010


The actual conversion algorithm used by the DVM is critical.
You actually need to use a standard integrating DVM with no deadtime 
between measurements or extra filtering.
Few DVMs actually have zero deadtime between measurements.
Some of the more recent ones may have.

Have you actually modeled the effect of a small constant frequency error 
or are you just hand waving?

WarrenS wrote:
>
> Yes I agree there are all kinds of ways to do it wrong, and some ways 
> to do it better,
>
> The original NIST uses a VtoF converter and then a counter to do the 
> Freq integration 'directly' (NO Phase info needed).
> This can all be replaced with a simple RC and a oversampling ADC and a 
> little knowledge.
>
> A simple DVM that updates and records at say two to 4 per second AND 
> has an internal  under 1 second filter TC,
> does the "average Freq over time" good enough NOT to be a problem, 
> when that data is then used for 1 sec and slower Tau.
>
> OK. SO need to use an RC pre-averager,  and need to be careful with 
> the sample rate and not 'push' it too far to get the "Direct 
> integration",
> But with just a little thought, you have NO aliasing and you get 
> perfect enough integration with no dead time using a oversampling.
> (I use 1 KHz over-sampling to do near perfect 10 Hz integration and OK 
> 100Hz integrated data.)
>
>> If one uses phase ...
> Easy yes, BUT, You failed to mention that to get the same sub 0.1ps TI 
> phase resolution and performance that the simple "Tight Phase-Lock 
> Loop Method" gives for free, will take a lot more than "easy".
>
> So if one of your points is that one has to be careful and know what 
> they are doing, ... We Agree.
> or that there are ways to do better given an unlimited budget ...We Agree
> Or this is not the simplest and cheapest way to get 1e13 resolution at 
> 1 sec... You have NOT yet commented on that so I don't know if we agree?
>
> Just so things do not get too far off the original topic that happens 
> all to often, here is a reminder:
>>>>> "I would appreciate any comments or observations on the SIMPLEST 
>>>>> scheme for making stability measurements at 1e-13 in one sec."
> ws Answer)  Try the "Tight Phase-Lock Loop Method"
>
> ws
>
> **************
>
> ----- Original Message ----- From: "Bruce Griffiths" 
> <bruce.griffiths at xtra.co.nz>
> To: "Discussion of precise time and frequency measurement" 
> <time-nuts at febo.com>
> Sent: Saturday, February 06, 2010 1:16 PM
> Subject: Re: [time-nuts] ADEV vs MDEV
>
>
>> The tight PLL method doesn't directly produce the average frequency over
>> Tau.
>> As explained in (see snapshot of relevant section):
>> NIST special Publication 1065 Handbook of Frequency Stability Analysis
>> <http://tf.nist.gov/timefreq/general/pdf/2220.pdf>
>>  the average frequency deviations for averaging time Tau are needed for
>> the calculation.
>> You need to sample at a sufficiently high rate to avoid aliasing and
>> average (ie integrate) the individual EFC samples.
>>
>> If one uses phase measures then the fluctuations in the frequency
>> averages can easily and directly calculated from the difference between
>> the phase measured at time intervals separated by Tau.
>>
>> Bruce
>>
>> WarrenS wrote:
>>> Bruce said:
>>>
>>>> Thus NIST and others quietly dropped this method several decades ago.
>>>    Could it be another reason?
>>> I'll bet that was after they wanted to do better than 1e14 resolution
>>> AND had unlimited amounts of time and Money,
>>> Something most time Nuts are not blessed with.  I Never said it was
>>> the BEST way.
>>> JUST given the goal, which was 1e13  in one second, there is not a
>>> simpler and cheaper way to do it.
>>> And nothing you said counter that point.
>>>
>>>
>>>> The frequency measures need to be integrated (either implicitly or
>>>> explicitly) to produce phase measures which can then be used to
>>>> calculate ADEV, MDEV etc.
>>>
>>>    Well ONE of us certainly has something backward.
>>> To calculate ADEV, MDEV etc. YOU need Freq Differences.
>>> The first thing that happens when phase is used is that it is turned
>>> into Freq by taking the difference between each sample.
>>> Integrated Freq data, which is what "Tight Phase-Lock Loop Method"
>>> gives you directly (no Phase conversion needed),
>>> Need not FIRST turned into Phase so that it can then be turned back
>>> into Freq.
>>> BUT in any case there is no difference in the noise, for a given
>>> bandwidth, If you don't run out of digits and You have enough 
>>> resolution.
>>> The "Tight Phase-Lock Loop Method" can EASY get sub pS resolution,
>>> which is better than most other ways.
>>> AND don't need filters and slue rate control and multistage limiters
>>> and on & on to do it, an RC works fine to replace all the stuff.
>>>
>>> ws
>>>
>>> *****************
>>>
>>> ----- Original Message ----- From: "Bruce Griffiths"
>>> <bruce.griffiths at xtra.co.nz>
>>> To: "Discussion of precise time and frequency measurement"
>>> <time-nuts at febo.com>
>>> Sent: Saturday, February 06, 2010 12:11 PM
>>> Subject: Re: [time-nuts] ADEV vs MDEV
>>>
>>>
>>>> Sounds good but you still haven't found its Achilles heel:
>>>>
>>>> The frequency measures need to be integrated (either implicitly or
>>>> explicitly) to produce phase measures which can then be used to
>>>> calculate ADEV, MDEV etc.
>>>> The major problem is that integration amplifies the small errors that
>>>> are inevitably present.
>>>> In practice (except for very noisy sources) the technique isnt
>>>> particularly useful for Tau more than a few times the inverse PLL
>>>> bandwidth.
>>>>
>>>> Thus NIST and others quietly dropped this method several decades ago.
>>>> This is alluded to in Steins recent paper availble on the Symmetricom
>>>> website:
>>>>
>>>> *The Allan Variance – Challenges and Opportunities*
>>>>
>>>>
>>>> Bruce
>>>>
>>>> WarrenS wrote:
>>>>> Peat said:
>>>>>
>>>>>> I would appreciate any comments or observations on the topic of
>>>>>> apparatus with demonstrated stability measurements.
>>>>>> My motivation is to discover the SIMPLEST scheme for making
>>>>>> stability measurements at the 1E-13 in 1s  performance level.
>>>>>>
>>>>>
>>>>> If you accept that the measurement is going to limited by the
>>>>> Reference Osc,
>>>>> for Low COST and SIMPLE, with the ability to measure ADEVs at that
>>>>> level,
>>>>> Can't beat a simple analog version of  NIST's "Tight Phase-Lock Loop
>>>>> Method of measuring Freq stability".
>>>>> http://tf.nist.gov/phase/Properties/one.htm#oneone    Fig 1.7
>>>>>
>>>>>
>>>>> By replacing the "Voltage to freq converter, Freq counter&  Printer
>>>>> with a Radio shack type PC data logging DVM,
>>>>> It can be up and running from scratch in under an Hr, with no high
>>>>> end test equipment needed.
>>>>> If you want performance that exceeds the best of most DMTD at low
>>>>> Tau it takes a little more work
>>>>> and a higher speed oversampling ADC data logger and a good offset
>>>>> voltage.
>>>>>
>>>>> I must add this is not a popular solution (Or a general Purpose one)
>>>>> but
>>>>> IF  you know analog and have a GOOD osc with EFC to use for the
>>>>> reference,
>>>>> as far as I've been able to determine it is the BEST SIMPLE answer
>>>>> that allows High performance.
>>>>> Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1
>>>>> sec (at 30 Hz Bandwidth)
>>>>>
>>>>> Basic modified NIST Block Diag attached:
>>>>> The NIST paper sums it up quite nicely:
>>>>> 'It is not difficult to achieve a sensitivity of a part in e14 per
>>>>> Hz resolution
>>>>> so one has excellent precision capabilities with this system.'
>>>>>
>>>>> This does not address your other question of ADEV vs MDEV,
>>>>> What I've described is just a simple way to get the Low cost, GOOD
>>>>> Raw data.
>>>>> What you then do with that Data is a different subject.
>>>>>
>>>>> You can run the raw data thru one of the many ADEV programs out
>>>>> there, 'Plotter' being my choice.
>>>>>
>>>>>
>>>>> Have fun
>>>>> ws
>>>>>
>>>>> *************
>>>>>
>>>>> [time-nuts] ADEV vs MDEV
>>>>> Pete Rawson peterawson at earthlink.net
>>>>> Sat Feb 6 03:59:18 UTC 2010
>>>>>
>>>>> Efforts are underway to develop a low cost DMTD apparatus with
>>>>> demonstrated stability measurements of 1E-13 in 1s. It seems that
>>>>> existing TI counters can reach this goal in 10s. (using MDEV estimate
>>>>> or 100+s. using ADEV estimate). The question is; does the MDEV tool
>>>>> provide an appropriate measure of stability in this time range, or is
>>>>> the ADEV estimate a more correct answer?
>>>>>
>>>>> The TI performance I'm referring to is the 20-25 ps, single shot TI,
>>>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data
>>>>> from my CNT81showing MDEV<  1E-13 in 10s. and I believe the
>>>>> other counters behave similarly.
>>>>>
>>>>> I would appreciate any comments or observations on this topic.
>>>>> My motivation is to discover the simplest scheme for making
>>>>> stability measurements at this performance level; this is NOT
>>>>> even close to the state-of-the-art, but can still be useful.
>>>>>
>>>>> Pete Rawson
>>>>>
>
>>
>
>
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